Although the bleed valve of the present invention may be configured as a bleed valve for either a hydraulic or pneumatic reservoir, it will be described primarily with reference to an air bleed valve for a pressurized hydraulic reservoir. Bleed valves of various types have been placed in reservoirs and fluid return lines of hydraulic systems. Many of these valves have been large and often have been manually operated. A compact, automatic bleed valve for such systems has been described in U.S. Pat. No. 4,524,793 to Silverwater, the inventor in the present application.
A general theory of automatic bleed valve operation is explained in Silverwater '793 which utilizes a capillary and orifice placed in series in a fluid channel to cause the pressure distribution along the channel between a high pressure point at the reservoir end of the valve and a low pressure point at the discharge end of the valve to vary depending upon the phase of the fluids flowing in the channel. This theory is based upon the known fact that, in such an arrangement, a steeper pressure gradient will occur over the orifice in the case of gaseous phase flow and, conversely, a steeper gradient will be observed over the capillary portion of such a channel during liquid phase flow. The variation in the pressure distribution in the channel may be utilized to control the opening and closing of a differentiating valve, depending upon the phase of flow through the valve, as is explained in the specification of that patent. The preferred embodiment disclosed in that patent is automatic and, thus, mitigates the need for constant operator vigilance, and is relatively compact, allowing versatility in placement of the valve in the system and reducing weight, features which may be particularly important in, for example, aircraft applications. However, the valve of that embodiment is also mechanically complex. Manufacture of valves such as in the preferred embodiment of that application is complicated by the need to accurately fabricate and assemble a number of interacting mechanical parts. Multiple springs and rolling diaphragm seals are present in such valves, increasing the risks of mechanical failure. Further, such a large number of interacting parts increases the potential for complications resulting from dirt contamination of the valve.
The present invention is a different mechanism from the earlier bleed valve and provides important additional safety features, such as system shutoff. In the preferred embodiment of the previous invention, a differentiating piston operates within a bore which is located in a second, actuating piston. The actuating piston, in turn, operates within a fluid channel to begin the bleeding process when the reservoir is pressurized during start-up of the hydraulic system. Having two cooperating coaxial pistons within a single chamber complicates fabrication and assembly of the valve, increases the number of sealing members required, increases weight of the bleed valve, and complicates fabrication of the valve assembly.